In these systems, the digital data are transmitted by modulation of a radiofrequency carrier wave. Stated otherwise, a radio signal is sent over the transmission channel, this signal being modulated so as to carry the digital information to be transmitted.
The expression “estimating the transmission channel” is understood to mean in a conventional manner estimating the conditions of propagation of the radio signal through the latter, which affect the signal transmitted.
One seeks to implement modulation techniques that offer better resistance with regard to disturbances undergone by the radio signal during its transmission through the transmission channel. In essence, these disturbances originate:                on the one hand from the fading phenomenon, which is frequency selective as soon as the coherence band is overstepped (one speaks in this first case of selective fading), but which is not frequency selective once the width of the channel is less than the coherence band (one speaks in this latter case of flat fading). This fading phenomenon is due to the propagation multipaths which give rise to intersymbol interference (ISI) also known as intersymbol distortion;        on the other hand, from the fact that the amplitude and the phase of the or of each of the propagation paths may be static (in the sense that they do not vary in the course of time) or on the contrary dynamic (when the propagation conditions vary in the course of time). In the dynamic case, the frequency of this phenomenon (also called the frequency of the fading) and, more generally, the frequency spectrum of the fading are related to the speed of the mobile and to the carrier frequency of the signal sent. The conventional model adopted for the power spectrum of the fading is described in the work “Microwave Mobile Communications”, by William C. Jakes, Jr., published by John Wiley & Sons, 1974, pp. 19-25), and involves the Doppler frequency fD given by:        
                              f          D                =                              V            c                    ×                      f            c                                              (        1        )            
where V is the speed of the mobile, c is the speed of light, and fc is the frequency of the radiofrequency carrier.
There is currently effort to seek to implement a multicarrier modulation called OFDM (standing for “Orthogonal Frequency Division Multiplexing”). This modulation technique has been adopted for the European standard regarding digital audio broadcasting systems (DAB systems, the abbreviation standing for “Digital Audio Broadcasting”). It consists in distributing the data to be transmitted over a set of subcarriers sent in parallel in the radio signal. This results in a flat fading effect in relation to each subcarrier since the bandwidth of each subcarrier is less than the coherence band. Furthermore, it results in a reduction in the sensitivity of transmission in relation to the phenomenon of multipaths.
The signal to be transmitted is constructed on a time/frequency lattice. Such a time/frequency lattice comprises a set of symbols, constituting a two-dimensional space which is defined by a frequency axis and by a time axis. It is recalled that a symbol corresponds to a determined number of information bits, for example 8 bits, which takes a determined value in an ad-hoc alphabet. By convention, the frequency axis is represented vertically and the time axis is represented horizontally. Each symbol is tagged by an index m along the frequency axis, and by an index n along the time axis. By convention, a symbol whose position along the frequency axis is defined by the index m, and whose position along the time axis is defined by the index n is in general denoted Sm,n. Finally, the spacing between the symbols along the frequency axis is denoted γ0. Likewise, the spacing between the symbols along the time axis is denoted τ0.
If S(t) denotes a signal constructed on such a lattice of symbols, the signal S(t) can be decomposed into the form:
                              S          ⁡                      (            t            )                          =                              ∑                          m              ,              n                                ⁢                                    c                              m                ,                n                                      ×                          ⅇ                              2                ·                ⅈ                ·                m                ·                                  γ                  0                                                      ×                          g              ⁡                              (                                  t                  -                                      n                    ·                                          τ                      0                                                                      )                                                                        (        2        )            
where the sign Σ designates the summation operation;
where the coefficients cm,n are coefficients corresponding to the value of the symbol Sm,n; and
where the function g(t) designates the shaping pulse for the modulation.
The signal to be transmitted is structured as frames that are transmitted in succession through the transmission channel. Each frame comprises a number M of adjacent subcarriers inside a channel of determined spectral width, each of these subcarriers being divided into N time intervals, called symbol times, which are transmitted in succession through the transmission channel. The duration of a symbol time corresponds to the duration of transmission of a symbol. A frame of the signal therefore comprises M×N symbols. The aforesaid parameter γ0 represents the spacing between two adjacent subcarriers, and the aforesaid parameter τ0 represents the spacing between two successive symbols on one and the same subcarrier.
In systems using OFDM type modulation, the shaping pulses for the modulation are chosen in such a way that each symbol is orthogonal with all the other symbols. The lattice is then said to be orthogonal. By definition, symbols are mutually orthogonal if their scalar product is zero.
This characteristic makes it possible to simplify demodulation.
Systems using OFDM modulation subdivide into two categories.
On the one hand, the systems using a time/frequency lattice of density 1 (subsequently referred to as “systems of density 1”, for short) for which the product γ0×τ0 is equal to unity (γ0×τ0=1). In these systems the modulated symbols may be complex symbols. The aforesaid coefficients cm,n are then complex numbers. We can write cm,n=am,n+i×bm,n, where am,n and bm,n are real numbers. This offers the possibility of employing both amplitude modulation and phase modulation. In practice, a guard must however be taken in the frequency domain and/or in the time domain between two consecutive adjacent symbols along the frequency axis, respectively along the time axis. This guard substantially reduces the maximum throughput (expressed as a number of symbols per second, or baud) which may flow through the transmission channel.
On the other hand, systems using a time/frequency lattice of density 2 (subsequently referred to as “systems of density 2”, for short) for which the product γ0×τ0 is equal to
      1    2    ⁢            (                                    γ            0                    ×                      τ            0                          =                  1          2                    )        .  In these systems, the maximum throughput (expressed as a number of symbols per second, or baud) is twice as high as in the systems of density 1. However, in systems of density 2, the modulated symbols must be one-dimensional, that is to say they either have a real value (one then speaks of real symbols), or a pure imaginary value (one then speaks of pure imaginary symbols). We can write cm,n=am,n for the real symbols or cm,n=i×bm,n for the pure imaginary symbols, where am,n and bm,n are real numbers. More precisely, if a symbol is real, its immediate neighbors, that is to say the symbols situated on the same subcarrier in the immediately previous and immediately subsequent symbol times (with reference to the order of sending of the symbols over the transmission channel, that is to say the symbols that are adjacent in the direction of the time axis) and the symbols that are situated in the same symbol time on the subcarriers placed on the immediately higher and immediately lower frequencies (i.e., the symbols that are adjacent in the direction of the frequency axis), are pure imaginary. Conversely, if a symbol is pure imaginary, its immediate neighbors, that is to say the symbols adjacent in the direction of the frequency axis and the symbols adjacent in the direction of the time axis, are real. Systems of density 2 do not require the presence of a frequency guard or time guard. They therefore make it possible to transport a higher throughput than systems of density 1.
In what follows, only the case of systems of density 2 will be considered. The invention applies in fact to systems of this type.
A particular example of an OFDM type modulation in a system of density 2 is so-called OFDM/IOTA modulation (the initials standing for “OFDM/Isotropic Orthogonal Transform Algorithm”). The way in which a time/frequency lattice that is orthogonal with such a modulation can be defined is described for example in the article “Coded Orthogonal Frequency Division Multiplex”, Bernard L E FLOCH et al., Proceedings of the IEEE, Vol. 83, No. 6, June 1995).
The coefficients cm,n are then either real numbers or pure imaginary numbers, depending on the placement of the symbol Sm,n in the frame. They are therefore always one-dimensional. This offers only the possibility of amplitude modulation. Nevertheless, it is not necessary to guarantee a guard time between the symbols or between the subcarriers, this having the advantage of increasing the transmission throughput.
Therefore, half the symbols transmitted are real and half are pure imaginary. These symbols are shaped by the modulation pulse g(t) mentioned earlier. This pulse extends over the time axis, over a duration corresponding to several symbols, and/or over the frequency axis, over frequencies corresponding to several subcarriers.
On receipt of a radio signal, a time and frequency synchronization of the signal received is performed. The signal received is then correlated with the signal expected, that is to say a correlation of the signal received with the modulation pulse g(t) is performed. This correlation may be performed by various procedures, for example by performing a multiplication by the modulation pulse g(t) then an FFT (standing for “Fast Fourier Transform”).
Thereafter it is appropriate to proceed with the estimation of the propagation conditions over the transmission channel, that is to say the estimation of the transmission channel, also called estimation of fading since it produces an estimated value of the fading of the signal transmitted through the transmission channel. Specifically, these propagation conditions have to be taken into account when demodulating the signal received, and more precisely when estimating the value of the symbols transmitted.